1. Field of the Invention
The present invention relates to a head box for a paper making machine, or the like, and particularly to such a head box having a flow channel in it which tapers towards its outlet opening. At least one guide surface is arranged in the channel. It extends across the entire width of the channel and is anchored in the channel.
2. Description of the Prior Art
Such a head box is known, for instance, from German Provisional Patent (Auslegeschrift) No. 1 761 229 and from U.S. Pat. No. Re. 28 269, the latter being incorporated by reference.
One guide surface may be provided in the channel. Alternatively, several guide surfaces arranged parallel to each other, can be provided. The guide surface or surfaces should be able to move freely in the head box under the influence of the flow of pulp. The pulp should thus not be conducted in rigid flow paths through the outlet channel. In this way, the turbulence present at the inlet might only be gradually destroyed. As a result, a very fine dispersion of the fibers in the fiber suspension or pulp suspension is to be obtained.
In general, a head box is charged with pulp suspensions having different respective compositions. The paper produced from the suspensions is therefore a multilayer product having individual layers of different compositions. The thickness of an individual layer of the paper produced can be determined in advance, on the one hand, by the shaping of the head box, and particularly the vertical distance apart of the guide surfaces, and, on the other hand, by the operating conditions of the head box i.e., the rate of flow and pressure of the individual streams of pulp suspension.
It is desired to maintain a constant thickness for each individual layer of the paper. It has, however, been found that this cannot be achieved to the required extent using head boxes of the type described above. Often, the thicknesses of the individual layers in a web of paper vary. Thus, it may, for instance, happen that in the case of a finished, multi-layer web of cardboard, a central layer will be undesirably evident.
One effort to overcome this problem has comprised attempting to adjust the pressures prevailing in the individual streams of pulp to specific values. However, this has not been of any substantial assistance.